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/*
* Copyright 2020 The Android Open Source Project
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
#pragma once
#include <future>
#include <type_traits>
#include <utility>
namespace android::promise {
namespace impl {
template <typename T>
struct FutureResult {
using Type = T;
};
template <typename T>
struct FutureResult<std::future<T>> {
using Type = T;
};
} // namespace impl
template <typename T>
using FutureResult = typename impl::FutureResult<T>::Type;
template <typename... Args>
inline auto defer(Args... args) {
return std::async(std::launch::deferred, std::forward<Args>(args)...);
}
template <typename T>
inline std::future<T> yield(T&& v) {
return defer([](T&& v) { return std::forward<T>(v); }, std::forward<T>(v));
}
template <typename T>
struct Chain {
Chain(std::future<T>&& f) : future(std::move(f)) {}
operator std::future<T>&&() && { return std::move(future); }
T get() && { return future.get(); }
template <typename F, typename R = std::invoke_result_t<F, T>>
auto then(F&& op) && -> Chain<FutureResult<R>> {
return defer(
[](auto&& f, F&& op) {
R r = op(f.get());
if constexpr (std::is_same_v<R, FutureResult<R>>) {
return r;
} else {
return r.get();
}
},
std::move(future), std::forward<F>(op));
}
std::future<T> future;
};
template <typename T>
inline Chain<T> chain(std::future<T>&& f) {
return std::move(f);
}
} // namespace android::promise
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